Conventionally, in an image forming apparatus such as a digital copying machine that uses a semiconductor laser as a light source, a surface of an image carrier has been subjected to exposure scanning by a line unit to form an electrostatic latent image. To maintain constant emission power of the semiconductor laser in an image area, emission power stabilization control (generally called Auto Power control; abbreviated to APC hereinafter) has been carried out.
According to the APC, power of a laser beam is detected by a photodiode incorporated in the semiconductor laser, or a photodiode externally, and a laser beam is emitted by desired power in accordance with a detected current from the photodiode. Specifically, first, a predetermined current is supplied to the semiconductor laser, and power of an emitted laser beam therefor is detected as a current by the photodiode. The detected current is converted into a voltage, and a voltage value (detected voltage Vm) after the conversion is compared with a voltage value (reference voltage Vref) for desired emission power. If the detected voltage Vm is larger than the reference voltage Vref, electric charges are discharged from a reference capacitor. If the detected voltage Vm is smaller than the reference voltage Vref, electric charges are applied to the reference capacitor. A current is supplied to the semiconductor laser in accordance with a terminal voltage of the reference capacitor. As a result, the detected voltage Vm is adjusted to be equal to the reference voltage Vref, and the power of a laser beam is maintained constant.
Additionally, the APC is classified into two: a bias APC (BAPC) which controls a bias current equal to/less than a laser emission threshold value, and a write APC (WAPC) which controls a switching current of laser emission turned ON/OFF in accordance with image data.
Jpn. Pat. Appln. KOKAI Publication No. 9-164722 discloses a technology which shortens a time necessary for APC during one scanning by executing APC of one light source during one scanning and APC of a plurality of light sources over a plurality of scanning operations in an apparatus which has the plurality of light sources.
However, the technology of the Publication has problems. That is, in a certain light source, emission power of a light during the scanning in which no APC is executed is smaller compared with that during the scanning in which APC is executed. This is caused by drooping, i.e., a leakage current, of a capacitor which holds a voltage value to decide emission power.
Normally, a leakage current of the holding capacitor is sufficiently small with respect to one scanning cycle. However, in the case of using a laser array which has many light sources, an influence of a leakage current can no longer be ignored. Consequently, density nonunifoirmity occurs between the scanning in which the APC is executed and the scanning in which no APC is executed.